The present invention relates generally to audio sound systems and more particularly concerns audio sound systems which can decode two-channel stereo into multi-channel sound, commonly referred to as “surround” sound. Typical prior art systems have utilized a variable output matrix for decoding a given signal into multi-channel outputs. Surround matrix systems capable of providing more than two output channels are well known. The Dolby Prologic® system is to date perhaps the best known example of a variable output matrix system that can decode a stereo encoded signal into four channels. For several years there has been a desire to increase the number of output channels in a matrix system to five or more. There has also been a desire to provide stereo performance in the rear surround channels. This is especially desirable when using a matrix system to decode non-encoded stereo music. U.S. Pat. Nos. 5,319,713 and 5,333,201 disclose a surround system which provide a stereo surround signal by steering a mono L−R signal in multiple bands. While this system will provide a stereo perception by steering dominant left or right signals in multiple bands, it lacks the finer detail or resolution of a true stereo signal. The surround system disclosed in U.S. Pat. No. 5,796,844 will provide a true stereo left and right surround signal when no dominant front center signal is present. When a dominant front center signal is present the '844 patent system reverts to mono in the surround channels or must compromise the front to rear separation of the center information. As a result, the '844 system frequently produces a mono signal in the surround channel outputs when there is a dominant front center signal. The left to right separation of the surround channels is one of the most important aspects of the surround system performance as perceived by the listener. The better the left/right stereo separation of the system, including the surround channels, the better the perceived performance of the system. In most of the matrix systems available today, the low frequency portion of the spectrum is dynamically changing when there is any active steering in the matrix. This will tend to produce subtle but noticeable instability at the bass frequencies. Furthermore, all of the matrix surround systems exhibit a noticeable increase in reverberation when decoding non-encoded stereo music compared to a stereo playback.
It is, therefore, a primary object of the current invention to provide a dynamic spectral matrix surround system which maintains maximum true stereo performance in the left and right front and left and right surround channels. It is also an object of the invention to provide a dynamic spectral matrix surround system which maintains true stereo operation in the high frequency portion of the spectrum when there is no high frequency center channel information present. A further object of the invention is to provide a dynamic spectral matrix surround system which affords maximum perceived removal of the front center signal in the left and right front and left and right surround channels while simultaneously providing maximum stereo separation. Another object of the invention is to provide a dynamic spectral matrix surround system which improves the stability of the bass frequencies during the dynamic steering of the matrix. Yet another object of the present invention is to provide a dynamic spectral matrix surround system that is compatible with all matrix encoded material, as well as all non-encoded stereo material. And it is an object of the present invention to provide a dynamic spectral matrix surround system which reproduces non-encoded stereo material with a more correctly balanced level of difference information, thereby reducing the typical increase of originally recorded reverberation.